Fourier transform visible and infrared difference spectroscopy for the study of P700 in photosystem I from Fischerella thermalis PCC 7521 cells grown under white light and far-red light: Evidence that the A–1 cofactor is chlorophyll f

The structure of Photosystem I acclimated to far-red light illuminates an ecologically important acclimation process in photosynthesis

“…While some of the pigments discussed here show clear structural evidence supporting their assignment, as Chl f and other possible sites are discussed above (also see fig. S7B), note that, consistent with the results from FTIR difference spectroscopy, the formyl groups of Chl f do not necessarily need to be H-bonded (27). Therefore, although the search for nearby H-bonding partners to the formyl groups of Chl f may provide clues as to which pigments could be Chl f (e.g., Chl B30), the absence of H-bonding partners does not exclude protein environments that cannot provide these interactions, which could be provided by water molecules not visible at this resolution and possibly explaining why we could only identify four of the expected seven Chl f molecules.…”

Breaking the Red Limit: Efficient Trapping of Long-Wavelength Excitations in Chlorophyll-f-Containing Photosystem I

“…5 The earlier proposed A20/A21 dimer, 30 in which A20 is suggested to be Chl f, appears to be too distant and too weakly coupled to the RC pigments to be a candidate dimer for the f790 pigments. Of note, a previous study observed a band shift attributed to a Chl f in the P 700 + -P 700 IR difference absorption spectra of FRL-PSI of FT7521, 44 advocating for the proximity of a Chl f molecule to P 700 . This observation led to the suggestion that a Chl f is located in the RC of FRL-PSI (at the A À1 position).…”

Opportunities and challenges for assigning cofactors in cryo-EM density maps of chlorophyll-containing proteins

“…Various investigations have used spectroscopic approaches to identify the specific Chl f sites within FRL-PSI 29 , 30 , 67 , 70 – 72 ; however, these have resulted in some conflicting assignments that will be discussed below. Figure 2 shows a monomer of PSI and each of the 12 positions that have been proposed to be Chl f in the current literature—although each monomeric complex should only contain ~7 Chl f molecules (8% of ~90 total Chls).…”

“…Figure 2 shows a monomer of PSI and each of the 12 positions that have been proposed to be Chl f in the current literature—although each monomeric complex should only contain ~7 Chl f molecules (8% of ~90 total Chls). Spectroscopic studies have provided evidence for 29 , 70 – 72 and against 30 the presence of Chl f in one or both A –1 sites in the electron-transfer chain (ETC), although neither of the two recent cryo-EM structures assigned any Chl f molecules in the ETC 2 , 3 . Gisriel et al 2 assigned four Chl f sites in the core antenna of the FRL-PSI structure from F. thermalis PCC 7521 and Kato et al 3 assigned seven Chl f sites in FRL-PSI from H. hongdechloris , but only one position was commonly identified in the two structures.…”